This has never happened before. When I right click on any track the box is ticked for 'Set as Default Height'. It's not greyed out so I can tick it, but the tick won't go away. I like to lock some tracks on the minimum height as I won't be needing to do anything with them, but on this project I can't.  Have I by come chance clicked something without meaning to that is now keeping this tick locked, i.e. so I am forced to have all tracks the same height?

Bump?

It is apparently telling you that the track is at the default height.  Change the height of the track by dragging, then right click and look again.  It will be unchecked automagically  :)